Investigation on the effect of alkali-free aluminium sulfate based accelerator on the fresh properties of 3D printable concrete

This study investigates the effect of the alkali-free aluminium sulphate based accelerator on the printability and buildability of 3D printable concrete. The accelerator is introduced at various locations in the printing system – at the mixer, nozzle and sprayed on the surface post-printing. The influence of accelerator on setting and hardening is examined using penetration resistance test and semi-adiabatic calorimetry test. Flow table test and vane shear test are used to determine the effect on static yield stress, structuration, and resistance to flow (spread), which determines the suitable dosage of the accelerator for a printable and buildable structure. The forced bleed test is used to determine the effect of the accelerator on the potential for phase separation and to determine the flowable free water in the accelerated mix. The test results are compared with the observations from print trials. Through print trials, it is concluded that the mix with 1% accelerator dosage provides better buildability when added at the mixer, and the 2% dosage is suitable for addition at the nozzle. A new method of enhancing buildability post-printing by spraying a high dosage of the accelerator is also studied. The higher stiffness of the sprayed surface due to the quick set (up to 5 mm depth) arrests compression of layers while printing. The depth of penetration and evolution of phases is examined by XRD analysis. A large-scale structure printed with this technique is demonstrated. © 2022 Elsevier Ltd